This invention relates to high voltage gas blast circuit breakers, and more specifically relates to a novel stationary contact structure for the interrupter of a circuit breaker.
Gas blast circuit breakers are well known in the art and one typical gas blast circuit breaker is shown in U.S. Pat. No. 3,909,571, dated Sept. 30, 1975, in the name of Hansruedi Aumayer, and assigned to the assignee of the present invention. In a two pressure circuit breaker of this type, a plurality of circuit interrupters are connected in series with one another and each of the circuit interrupters includes a stationary contact defined by a cluster of flexible contact fingers which are engaged at their interior end by a movable contact rod.
In order to open the interrupter, the movable contact rod is withdrawn from the stationary contact finger cluster and the arc drawn between the two separating contacts is extinguished by a blast of electronegative gas, such as sulfur hexafluoride, which is released by the opening of a blast valve at the time the contacts open.
The stationary contact of the interrupter described in the above U.S. Pat. No. 3,909,571 is formed of a conductive cylinder which may be slightly conical in cross-section, and which is slotted from one end to define a circular cluster of individual contact fingers. Each finger is seperated from the other by a distance defined by the thickness of the slot used to form the individual fingers, which slot typically is defined by the saw cut thickness which forms the individual fingers. This slot thickness typically may be about 1/16 of an inch. The interior diameter of the ends of the individual contact elements is smaller than the external diameter of the movable contact which engages these finger ends. Therefore, when the movable contact enters the cluster of stationary contact fingers, the fingers are pressed outwardly, for example, by about 1/32 of an inch, so that each finger engages the stationary contact with substantial pressure created by the elastic forces tending to maintain the individual fingers in their unstressed position.
When the circuit breaker described above has been used for 63 kA interruption duty, it was found tht during arcing, one or more of the fingers could deform inwardly due to the forces created by the parallel arc currents flowing in adjacent fingers which tends to pull the fingers together. The deflection of the contact fingers over the full slot thickness caused the fingers to flex beyond their elastic limit, and permanently distorted the fingers, with their saw cut gaps being relatively closed at the outer ends of the fingers. When the fingers were permanently distorted inwrdly in this manner, the circuit breaker movable contact was unable to reclose since the circle formed by the stationary contact fingers was now too small to be entered by the movable contact.